When grid power fails, a microgrid can only support a fraction of its normal load. Load shedding is the practice of dropping lower-priority circuits in stages so the critical ones stay energised as long as possible. This calculator groups your circuits into priority tiers, sums the load in each, and shows how long your battery or generator will last at each shedding stage.
The three-tier model
Effective load shedding starts with a clear priority classification. Think of it as three concentric rings:
Critical (never shed): These loads must run regardless of how long the outage lasts. Examples include medical equipment (CPAP, oxygen concentrators, infusion pumps), life-safety systems (fire alarm panels, emergency lighting, security), communication infrastructure (network switches, UPS), and refrigeration for medication or perishable goods.
Essential (shed only when critical runtime is in jeopardy): Loads that matter a great deal but can tolerate interruption if the alternative is losing the critical ring. Typical examples: sump pumps, well pumps, a single room’s HVAC, key lighting circuits, and a small number of power outlets.
Deferrable (shed first): Large discretionary consumers that can be stopped with no immediate safety or comfort consequence. EV chargers, electric water heaters, pool pumps, dryers, and secondary HVAC zones fall here.
How the runtime calculations work
Every circuit is tagged into a tier. The tool sums wattage per tier and builds three cumulative scenarios: critical only, critical plus essential, and all circuits. For each it computes runtime from your chosen energy source.
Battery bank:
usable_Wh = Ah × V × (DoD% / 100)
runtime_h = usable_Wh / load_WGenerator: Burn rate rises with load. The calculator scales rated consumption by load fraction with a 30% idle floor, and flags any stage that exceeds the generator’s continuous rating as an overload:
burn_now = burn_rated × (0.3 + 0.7 × load_fraction)
runtime_h = fuel / burn_nowWorked example
A 48V, 200 Ah lithium bank at 80% usable depth of discharge stores 200 × 48 × 0.8 = 7,680 Wh.
| Stage | Load | Runtime |
|---|---|---|
| Critical only | 400 W | 7,680 ÷ 400 ≈ 19.2 h |
| Critical + essential | 1,150 W | 7,680 ÷ 1,150 ≈ 6.7 h |
| All circuits | 3,200 W | 7,680 ÷ 3,200 ≈ 2.4 h |
The same calculation for a 5 kW generator at 0.5 gal/hr rated consumption running critical-only (400 W load fraction 0.08) yields a very low burn near idle — the run time is fuel-constrained, not watt-constrained.
Practical guidance
- Plan the order of shedding, not just the tiers. Draw up a manual switch-off list tied to your breaker labels before an outage happens, not during one.
- Account for motor start-up surge. Pumps and compressors can draw 3–6× their steady-state watts at startup. The generator rating must accommodate this even if the running watts are fine.
- Lead-acid depth of discharge. Keep lead-acid above 50% (80% for AGM in some designs) to avoid shortening cycle life. The 80% figure in the example assumes lithium chemistry.
- Verify your islanding scheme. Transfer switches and anti-islanding settings must comply with local utility interconnection rules before your system can legally operate during an outage.